Pollinator rarity limits reintroduction sites in an endangered sexually deceptive orchid (Caladenia hastata): implications for plants with specialized pollination systems

Establishment of reintroduction sites and supplementation of small populations is critical to the recovery of many threatened plant species. For plants with specialized pollination strategies, understanding the spatial distribution of pollinators is essential for successful reintroductions. For the critically endangered Caladenia hastata (Orchidaceae), we aimed to identify the pollinator and mechanism of pollination, test for the presence of the pollinator within potential reintroduction sites and determine reintroduction sites based on pollinator presence and vegetation composition. Baiting trials at existing populations confirmed pollination by sexual deception of the thynnine wasp Lestricothynnus hastata. Surveys of 233 sites showed the pollinator to be extremely scarce in the range of potential reintroduction sites and no longer present at two of the four extant populations of C. hastata. The pollinator was present at five of the 233 sites of which only two sites had similar vegetation structure and composition to extant populations of C. hastata containing the pollinator. Caladenia hastata was reintroduced to two of these sites, with natural pollination and seed set subsequently confirmed. Restricted pollinator distribution in the former range of C. hastata is the key factor limiting reintroduction of this species. We have shown that an essential first step prior to reintroduction for plants with specialized pollination systems is to understand the presence and distribution of the pollinator.